Fluid responsiveness is difficult to assess at the bedside. The accuracy of published techniques to detect preload-dependent patients have many pitfalls and limitations. The present study test the role of noninvasive effective pulmonary blood flow measured by expired carbon dioxide to detect fluid responsivess in mechanically ventilated patients.

This is a prospective and observational study designed to test the accuracy of the non-invasive effective pulmonary blood flow measured by the capnodynamic methodology for detect preload-dependent patients.

Fourty patients undergoing mechanical ventilation during surgery will be studied. Preload-depency (fluid responsiveness) will be tested during an increase in end-expiratory pressure (PEEP) from 5 to 10 cmH2O during one minute. Pulse pressure variation will be use as the reference method to detect preload-dependency. The effective pulmonary blood flow will be continuously recorded during the PEEP maneuver. Receiver Operator Curves will be used to detect fluid responsiveness taking a pulse pressure variation higher than 13%.The corresponding cut off value for the effective pulmonary blood flow signal will be determined..

Inclusion Criteria:

• Written informed consent
• Programmed cardiac and noncardiac surgeries
• Need of invasive arterial blood pressure monitoring.

Exclusion Criteria:

• Emergency surgeries.
• Acute pulmonary diseases
• Arrhytmias
• Congestive cardiac failure

Cardiac and noncardiac surgery patients ASA 2-4